Early life social experiences and dysregulation of the brain reward system: age- and sex-dependent interactions

Early life environmental factors are critical for future wellbeing and development of appropriate social and cognitive skills. Exposure to a socially impoverished/socially enriched environment during the early stages of postnatal life may increase/decrease the salience of rewarding stimuli by facilitating/attenuating the risk of dysregulation of the brain reward system. The Communal Nesting (CN) paradigm is a form of pre-weaning environment enrichment in which three female mice are mated with the same male, gestate and rear their litters together until weaning. The Early Social Isolation (ESI) paradigm is an environmental manipulation consisting in a 30-min period of social isolation from both mother and peers during the 3rd postnatal week. In this study we trained adolescent and adult male and female rats reared at the different condition (SH, CN, SH+ESI, CN+ESI) to self-administered palatable food under a continuous and progressive reinforcement schedules. Results showed that CN rats took longer to acquire operant behavior and showed lower food intake than SH animals, with ESI leading to a steeper curve in adulthood. In general, SH-CTRL males showed higher food self-administration than corresponding females under FR-1 protocol, with ESI increasing the responding for food in SH adolescent females, but not males, and CN reverting ESI-induced effect. Under PR protocol, sex differences were more evident, as SH-ESI females showed a higher breaking point than SH-ESI males, and CN prevented the effect of ESI in both sexes. These findings indicate for the first time that an early life socially enriched condition, like the CN condition, exerts a "protective" effect toward early stress-induced behaviors related to reward and motivation.